Spencer Stock e-mailed his recollections of first hearing The Hum as a child living in Kettering in the 1940s. While his parents dismissed it as simply the sound of his own heart beating, Mr Stock found that only relocation to another area succeeded in freeing him of the all-pervasive sound.

He recalls, however, that the rooms in which he heard The Hum both had similar-sized chimneys, and wonders if an effect akin to blowing over the top of a bottle might be involved.

Mr H, a sound recordist for the BBC who prefers to remain anonymous, wrote to describe his own experiences of The Hum while making a documentary about it in the early 1990s. Those interviewed believed that the most likely source was the mains gas system, in whose pipes a standing wave formed as a result of friction between the gas and the pipe walls.

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Apparently, measurements made during the programme pointed to a frequency of 17 cycles a second, and a similar figure emerged from estimates of the "organ pipe effect". So perhaps this explains at least some cases of The Hum.

While working as an electrical engineer, Syd Porter of Whitstable reports that he has found several sources of humming sounds, from transformer sub-stations to wind blowing across overhead wires. "My own house was at one time plagued by a loud hum which turned out to be from a tightly strung telephone connection attached to the wall of the house, which was acting as a sounding board," he reports. "When the linesmen slackened the tension the noise disappeared."

Mr Porter also wonders if aircraft could be involved - a possibility raised by a fascinating anecdote provided by Ivan Beggs of Canton, Ohio. Both he and his son have often heard The Hum, and both noticed it disappear for a few days in 2001 - following the attack on the World Trade Centre.

"We realised that all the planes had been grounded," says Mr Beggs. "For the next couple of days, we noticed the same thing: no hum. Then the planes were allowed to fly and the hum returned." Could it be that The Hum "hot spots" in London and Bristol and Largs in Scotland are connected with intercontinental air traffic passing overhead? The accounts of several readers lead me to suspect at least part of the explanation lies much closer to home.

Malcolm Mourant of Bromley, Kent, described how he heard The Hum for several years, until he was diagnosed with high blood pressure and was prescribed pills to control it. "From then on, The Hum disappeared," he said. Dave Thacker of Bognor Regis has had a similar experience. "I noticed that The Hum had a regular rhythm, like a heartbeat. Next time I heard the sound, I felt my pulse and realised that the sound I was listening to was of the blood circulating round my body."

A quick check of the medical literature reveals the existence of a phenomenon called "venous hum". There are many causes, ranging from high blood pressure to disorders of the Eustachian tube or middle ear, which amplify the sound of the circulation system. Metabolic disorders such as anaemia or thyroid problems, which affect the flow of blood, can also be involved.

I am beginning to believe that The Hum has a variety of causes. Even so, those afflicted should certainly consider seeing a GP to rule out a medical origin for their experience of this distressing phenomenon.